A fan equipped to cool a vehicular radiator or an air conditioning condenser is generally formed by resin molding.
For the fan of this type, the shape and dimensions thereof have been designed focusing on enhancing the performance. However, because of the manufacturing method of resin molding, if the wall thicknesses of fan parts increase, the cooling after resin molding takes much time, which leads to a decrease in production efficiency. Therefore, it is preferable that the wall thicknesses of fan parts be as small as possible. The same holds true for reduction in weight. On the other hand, the decrease in wall thickness leads to a decrease in fan strength. As far as the product quality goes, a strength sufficient to prevent breakage or the like from occurring after the start of service is required. Thus, it is a matter of course that it is unpreferable to decrease the wall thickness indiscriminately.
Conventionally, in order to form such a fan by resin molding, resin is injected from gates formed at a plurality of locations of a mold, and the mold is released after the completion of the resin injection. At this time, resins injected from different gates join together in the mold. This joint of resins is called a weld. Comparing the weld part with other parts, the mechanical strength of the weld part is lower than those of other parts. If a weld is present in a stress concentrated portion, damage such as a crack occurs easily in the weld part.
To overcome this problem, there has been proposed a technique in which the wall thickness of the weld part is made larger than those of other parts to improve the mechanical strength of the weld part and to make damage less liable to occur (for example, refer to Patent Document 1). Also, there has been proposed a technique in which the weld is formed in a portion other than stress concentrated portions (for example, refer to Patent Document 2).    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2003-94494    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2007-285135